Infinitely-variable gearboxes, which are also called CVT-gearboxes (continuously variable transmission), are based on the principle of belt drives, in which the transmission ratio between the shortest and longest transmission ratio is infinitely-variable with the aid of a so-called variator. An infinitely-variable ratio of this type is best suitable for an optimum exploitation of the torque produced by the engine. As the ratio is infinitely-variable, an adequate ratio is always provided for an efficiency-oriented driving characteristic, but also for a consumption-oriented or sporty driving characteristic, in which the engine can work in the optimum operating range.
The variator consists of two pairs of cone pulleys, the primary set of pulleys and the secondary set of pulleys as well as of an enlacement means, which for instance can be formed as a plate link chain and which serves as a force transmission element. The primary set of pulleys is driven by the engine, the engine torque is transferred via the chain to the secondary set of pulleys and is guided into the axle drive. One cone pulley each is moveable on a shaft, whereby the run diameter of the chain and thus the ratio is infinitely-variable. The changes of the ratio are made jerk-free and without any interruption of the tractive force.
An automatic gearbox with the above discussed general conventional features is described in the article “Multitronic—the new automatic gearbox of Audi” (ATZ Automobile technical magazine, 2000, edition 7/8 and 9). This CVT-gearbox can be operated either in a constant speed mode, or in an acceleration mode. In the constant speed mode depending on the driving situation a fixed desired revolution speed is predetermined, which depends for example on the speed or the inclination.
In the acceleration mode with a constant position of the acceleration pedal the desired revolution speed is continuously increased, the rise of the increase being dependent on the driving situation. The driver can effect infinitely-variable increases or the reduction of the revolution speed, respectively, via the change of the angle of the acceleration pedal. If the gearbox is operated in the acceleration mode, the speed of the driving motor in the speed follow-up is continuously increased in infinitely-variable manner. Although this gearbox has proved itself in practice, it is partially considered to be disadvantageous that the gearbox behaves different in the acceleration mode than conventional stepped automatic transmissions.